(i) Field of the Invention
The invention relates to an improved hydrometallurgical process for the recovery of cobalt and nickel from nickel cobalt sulphides. More specifically, the invention relates to the separation of cobalt and nickel from an ammoniacal ammonium sulphate leach liquor to produce a substantially nickel-free cobalt solution from which recovery of high purity cobalt metal may be obtained and most particularly, to the treatment of the ammoniacal leach residue to obtain recovery of cobalt metal values contained therein and thus obtain overall enhanced cobalt recovery.
(ii) Description of the Related Art
A hydrometallurgical process for the treatment of nickel-cobalt-copper sulphide concentrates and mattes to produce high grade nickel and cobalt powders has been in commercial operation for many years. In this process, which is described in the publication, "The Winning of Nickel", J. R. Boldt, Jr. (Ed), Toronto, Longmans, Canada Limited, 1967, pages 299 to 314, the nickel sulphide feed materials are leached in ammoniacal ammonium sulphate solution, under elevated air pressure, in horizontal autoclaves. The metals, nickel, cobalt and copper are dissolved as metal amine complex ions. Iron is oxidized to insoluble haematite, and sulphide sulphur is partially oxidized to form a range of unsaturated sulphur anions which remain in the leach solution. The haematite residue is filtered off and discarded, and the leach solution is treated to remove copper, to partially remove the ammonia, and to oxidize the sulphur anions to sulphate. The purified solution, which contains about 60 g/L Ni and 1 g/L Co, is then treated with hydrogen under elevated pressure and temperature to precipitate nickel selectively as the metal powder. The cobalt remains in the reduced solution which contains about 1 g/L Ni and 1 g/L Co. The residual metals are then precipitated using hydrogen sulphide to produce a mixed nickel cobalt sulphide which forms the feed to the cobalt refining process.
The recovery of cobalt, in this nickel refining process, is much lower than that of nickel, as a result of the adsorption of cobalt, by the iron oxide leach residue. Furthermore, the selectivity of the reduction process, for nickel, depends on the maintenance of a low level of cobalt in the purified leach solution. It is only possible to produce nickel powder meeting the market specifications for cobalt, if the Ni:Co ratio is maintained above about 20:1. It will be appreciated, therefore, that this nickel refining process is not economically effective for the treatment of nickel feed materials with high cobalt content, since significant losses of cobalt to the leach residue and to the nickel powder product will be incurred.
A commercial cobalt refining process based on the soluble cobaltic pentammine process for separating nickel from cobalt, was operated in conjunction with the above nickel refining process. This process was based on U.S. Pat. Nos. 2,694,005; 2,694,006; 2,767,054 and 2,767,055 to Schaufelburger. In the refining process mixed nickel-cobalt sulphides with a typical Ni:Co ratio of 1:1 were first leached at elevated air pressure and temperature in dilute sulphuric acid solution to dissolve the nickel and cobalt and oxidize the sulphide to sulphate. The acidic leach solution, containing cobalt and nickel sulphates, was purified to remove iron and trace metals such as Cu, Cd and Zn. Ammonia was then added to neutralize the free acid and to adjust the ammonia to metals mole ratio to form the pentammine ions of divalent nickel and cobalt. The solution was then treated with air under pressure in an autoclave, to oxidize the cobalt (II) pentammine ion to the cobalt (III) pentammine ion. The nickel (II) pentammine ion is not oxidized under these conditions. The oxidized solution was then treated in a two stage process with concentrated sulphuric acid to selectively precipitate nickel as the nickel ammonium sulphate double salt (NiSO.sub.4 (NH.sub.4)SO.sub.4), leaving cobalt (III) pentammine sulphate in solution. After the second stage of nickel removal, the cobaltic solution was essentially nickel-free, with a Co-Ni ratio greater than 1000:1. The cobalt (III) ion was reduced to cobalt (II) and acid was added to adjust the NH.sub.3 :Co mole ratio to about 2:1. This solution was treated with hydrogen at elevated temperature and pressure to produce cobalt metal powder containing less than 0.1% Ni. This two-stage nickel removal process, as described in U.S. Pat. No 2,822,264, produced cobalt powder with a Co:Ni ratio of greater than 1000:1.
This cobalt-nickel separation process is particularly suitable for the treatment of mixed nickel-cobalt sulphides with Ni:Co ratios of about 1:1. As the nickel content of the sulphide increases, the process becomes less viable technically and economically, since the amount of nickel double salt to be precipitated and recycled to the nickel circuit increases, whilst the cobalt concentration in the purified solution decreases. In practice, a Ni:Co ratio of about 3:1 is the maximum that can be handled in this process. It will be appreciated, therefore, that neither ofthe above-described existing nickel-cobalt sulphide refining processes is suitable for the treatment of nickel-cobalt sulphides with Ni:Co weight ratios in the range 3:1 to 20:1.
When limonitic nickel laterite ores are treated by high temperature sulphuric acid pressure leaching, the dissolved nickel and cobalt are both recovered in high yields as a mixed sulphide typically containing about 55% Ni and 5% Co, i.e. with a Ni:Co ratio of about 10:1. This process is expected to become increasingly important economically in the future as the nickel mining industry switches to the treatment of laterite ores, as economically viable sulphide ore reserves are depleted. Material of this composition cannot be treated economically by either of the existing hydrometallurgical processes described supra.
Kerfoot, in U.S. Pat. No. 5,468,281, broadly teaches a process for producing cobalt powder from nickel-cobalt sulphides which involves precipitating the triple salt of cobalt (III) hexammine sulphate, nickel (II) hexammine sulphate and ammonium sulphate. More specifically, the nickel-cobalt sulphides are pressure leached in an ammoniacal ammonium sulphate solution at a temperature of at least 80.degree. C. at an effective ammonia to metals molar ratio ranging between 5:1 to 6.5:1 to oxidize the nickel and cobalt sulphides to sulphates thereby producing an ammoniacal leach liquor in which dissolved cobalt is predominantly in the (III) oxidation state, and a leach residue. The leach liquor is separated from the leach residue. The leach liquor is then saturated with an effective amount of anhydrous ammonia and cooled to below 50.degree. C. to thereby precipitate the triple salt comprising cobalt (III) hexammine sulphate, nickel (II) hexammine sulphate and ammonium sulphate. The leach liquor is passed to a nickel recovery circuit. The precipitated triple salt is recovered from the leach liquor and repulped with water to selectively leach nickel (II) hexammine sulphate and to produce a crystalline cobaltic (III) hexammine sulphate having a Co:Ni ratio of at least 100:1 and a nickel enriched leach liquor. The cobaltic (III) hexammine sulphate is recovered, dissolved in hot ammonium sulphate solution and cooled to precipitate recrystallized cobaltic (III) hexammine sulphate having a Co:Ni ratio of at least 1000:1, which is subsequently treated to produce cobalt powder therefrom.
This process has been in commercial operation for several years, having replaced the soluble cobaltic pentamine process described supra. Experience has shown, however, that in certain circumstances the losses of oxidized cobalt values in the ammoniacal ammonium sulphate leach residue, can vary widely in the range between 1 to 30 weight percent. Heretofore, efforts to recover the precipitated cobalt values using conventional wash solutions containing free ammonia and ammonium sulphate solution have proved unsuccessful.